Conventionally, it is known an electroluminescence display (hereinafter, referred to as an ELD) as an emission type electronic display device. As a composing element of an ELD, there are cited an inorganic electroluminescence element (hereinafter, referred to as an inorganic EL element) and an organic electroluminescence element (hereinafter, referred to as an organic EL element).
An inorganic EL element has been utilized mainly as a flat light source, however, it requires a high voltage of alternating current to drive an emission element. On the other hand, an organic EL element is an element provided with a constitution comprising an light emitting layer containing a light emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a hole being injected into the light emitting layer to be recombined, resulting light emission utilizing light release (fluorescence or phosphorescence) at the time of deactivation of the exciton; the light emission is possible at a voltage of approximately a few to a few tens volts, and an organic EL element is attracting attention with respect to such as wide viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
Moreover, an organic EL device has a distinctive feature of being a surface light, which is different from the main light sources, for example, a light-emitting diode and a cold cathode tube having been conventionally used. As applications which can effectively utilize this property, there are a light source for illumination and a back light of various displays. Especially, it is suitable to use as a back light of a full color liquid crystal display the demand of which has been increased remarkably in recent years.
When an organic EL element is used as a back light of a display or a light source for illumination as described above, there are required to have high light emission efficiency and a long emission lifetime. As a means to improve emission efficiency and to realize a longer emission lifetime, there is proposed a method in which organic functional layers (hereafter they are called as organic layers) composing an organic EL element are prepared by laminating a plurality of layers each having a different function. For example, it can be cited a layer composition of: hole transport layerlight emitting layer/electron transport layer/electron injection layer.
Moreover, the material used as a cathode of an organic EL element is required to inject many electrons into a light emitting layer. Therefore, a metal, such as aluminum, magnesium, indium, silver, or copper, is used so that it may be easy to carry out electron injection to the lowest unoccupied molecular orbital (LUMO) energy level of an organic material.
Furthermore, as a means to improve the electronic injection efficiency from a cathode to an organic layer, there is widely performed the following: preparing a layer containing an alkali metal or an alkali earth metal having a small work function, or a compound having these metals, between a cathode and an organic light emission layer. However, an alkali metal and an alkali earth metal, in particular, an alkali metal has a high reactivity, and there was difficulty in handling at the time of production of an organic EL element and in stability after the production of an organic EL element.
Against the above-mentioned problems, there was disclosed a technology to improve the light emitting property by providing with an electron injection layer composed of an alkali metal compound such as an oxide, a halide or a nitride, the electron injection layer being located at the place adjacent to the side of a cathode facing to an organic layer (for example, refer to Patent document 1). Since a chemically more stable alkali metal compound than an alkali metal is used with this technology, handling is easier and it is possible to produce an element with high reproduction stably using a vapordepositing method, for example. However, although it improves to some extent a luminescent property and a lifetime, still it is insufficient. On the other hand, it has problems that deterioration of efficiency and deterioration of lifetime will occur under a high temperature condition, or during a prolonged storage time. It is supposed as a cause that an alkali metal compound will diffuse to a light emitting layer under a high temperature condition or during a prolonged storage time.
As a means to improve the above-described situation, there were proposed the following technologies: an organic EL element provided with an electron injection layer containing an electron injection material and a first metal atom, and an electron transport layer containing an electron transport material and a second metal atom, the second metal atom being difficult to diffuse compared with the first metal atom, thereby the diffusion of the first metal atom is prevented (for example, refer to Patent document 2); an organic EL element provided with a transition preventing layer containing an organic compound which inhibits the passage of an electron donating material, the transition preventing layer being located at an interface of the light emitting layer facing the electron injection layer mixed with the electron donating material (for example, refer to Patent document 3); an organic EL element provided with a dopant diffusion preventing layer containing a metal fluoride and an organic conductive material, the dopant diffusion preventing layer being located between the electron injection layer doped with electrons and the light emitting layer (for example, refer to Patent document 4); and an organic EL element provided with an electron injection layer composed of a first electron injection layer and a second electron injection layer, the first electron injection layer containing an organic compound, and an alkali metal, an alkali earth metal or a compound of these metals, and the second electron injection layer containing and an alkali metal, an alkali earth metal or a compound of these metals (for example, refer to Patent document 5). All of these technologies are concerned with: a method to provide an interlayer containing an organic compound having a diffusion preventive property so as to prevent diffusion of an electron injective dopant (such as an alkali metal) from the electron injective region; or a method to prevent diffusion by arranging the position of a dopant containing layer in considering of the diffusion easiness or difficulty of an electron injective dopant. Although it can improve, to some extent, deterioration of performance under a high temperature condition, or during a prolonged storage time, still it is insufficient. There is a case in which occurs an increase of a driving voltage depending on the composition of a diffusion preventing layer, an organic material and an electron injective dopant. It is still difficult for a current status to reconcile the performance and the stability of an organic EL element.
Further, in the organic EL element which is subject to a heat aging treatment for the purpose of improving the stability of the performance, or in the organic EL element which is subjected to heat curing using a heat curing resin with a sealing member known as one of the promising method to realize a large sized element and to improve production efficiency, it is required to provide an organic El element which can suitably control the thermal diffusion of the electron injective dopant and can exhibits a stable emission property with a low driving voltage and with a high emission efficiency and long lifetime.